1. what are you interested? Museums (which I doubt since you have a 2 year old or maybe you could take turns?), major sights only?
2. Depends on where you live, flights could range from $1200 - $1800/each in May time frame
3. Hotels are cheaper in London while much more expensive in Paris, I think you should try to stay in the city center since you have little ones and seniors to travel with.